Polymer-impregnated sheet material, such as fabrics, films, paper and tapes, have been widely employed to form electrical insulation for various electrical equipment and components, including high voltage stator bars of a generator. Formation of such insulation generally involves the use of a pre-impregnated sheet material, often referred to as a prepreg, that can be applied directly to a member to be insulated. Various materials can be employed as the sheet material and the impregnation material, depending on the requirements of the applications.
As taught by U.S. Pat. Nos. 3,812,214, 4,603,182 and 4,656,090 to Markovitz, each assigned to the assignee of this invention, a prepreg of mica paper backed with a woven fabric backer, such as woven fiberglass, is often used in the manufacturing of high voltage stator bars. The mica paper can be employed with a single backer or in combination with two backers, in which one backer can be a woven fabric such as fiberglass while the second can be another woven fabric, a non-woven fabric such as a polyester mat, or a polyester or polyimide film. In each case, an epoxy resin binder is used to permeate through the mica paper and the backers, and to bond each backer to the mica paper so as to form a prepreg sheet.
Prepregs of the type taught by Markovitz are typically slit into tapes that can be more readily wrapped around a conductor, such as a stator bar of a generator. Typically, multiple layers of tape are tightly wrapped around the conductor, usually overlapping by one-half the width of the tape. After being wrapped with a sacrificial release film to protect the tape and prevent contamination, the conductor and its tape wrapping are then placed in an autoclave for vacuum heat treatment and subsequent curing. Vacuum heat treatment is carried out to remove air, moisture and any solvent or volatile compound present in the resin binder, so as to prevent formation of voids in the cured insulation that would otherwise adversely affect the quality of the insulation and induce premature insulation failure due to breakdown under electrical stress. Thereafter, the taped conductor undergoes a cure under pressure to consolidate the tape insulation, such that the resin binder bonds the mica paper and each of its backers together to form a void-free solid insulation.
In order to reliably form a high quality insulation, several requirements must be met in the manufacture and processing of resin-impregnated sheet materials, such as the mica tape and the taped conductor noted above. Preferably, the resin binder is a semi-solid or solid at room temperature, yet sufficiently flexible to make the sheet material pliable. Furthermore, the binder must have a sufficiently high molecular weight to act as an adhesive for bonding the prepreg components together, and must be substantially tack-free to prevent the prepreg from sticking together (i.e., blocking).
In addition, during the manufacture of the prepreg, the resin binder must be able to permeate through the sheet materials, as well as act as an adhesive to the one or two backers used in the construction of the prepreg. One known approach is to add a solvent, such as methyl ethyl ketone, acetone or toluene, to a semi-solid or solid resin so as to reduce its viscosity. While this approach is effective, a shortcoming is the relatively large amount of solvent that must be removed during the subsequent vacuum heat treatment of the taped conductor, an amount that is typically much greater than the moisture content of the tape. As noted previously, if the solvent is not completely removed, the retained solvent will adversely affect the cured properties of the binder and can promote the formation of voids in the cured insulation. An additional shortcoming of the use of solvents is the environmental and safety concerns associated with their use.
An additional requirement is that, during processing of a conductor wrapped or taped with the prepreg, the vacuum heat treatment must sufficiently lower the viscosity of the resin binder within the sheet material and increase the vapor pressure of its volatile compounds, so as to enable the removal of the volatile components. Such a requirement is particularly important in the use of multiple, tightly-wrapped layers of mica tape. For example, vacuum cycles of at least about five hours and often up to about twelve hours, at temperatures of up to about 120.degree. C, are typically required to remove the volatile compounds from the multiple tape layers around a stator bar. However, a significant problem with this step is the tendency for the resin binder to be reactive at the vacuum heat treatment temperatures necessary to adequately reduce the viscosity of the binder so as to remove the volatiles. As a result, the binder will begin to gel, particularly if the temperature is too high, the duration of the cycle is excessive, or if the prepreg is aged such that the resin binder has already begun to react.
Finally, the resin binder must be able to flow under pressure during the curing stage in order to fill all voids between the prepreg layers and between the prepreg and the conductor. However, if gelation has occurred during the vacuum heat treatment cycle, there will be insufficient resin flow during cure, such that voids will likely remain and degrade the effectiveness and the life of the cured insulation. While the reactivity of the binder could be reduced in order to prevent gelation during vacuum heat treatment, the result has been a reactivity which is inadequate to achieve sufficient cure during the cure cycle within a practical process cycle of about twelve hours at about 165.degree. C.
While the above-noted U.S. Pat. Nos. 3,812,214, 4,603,182 and 4,656,090 to Markovitz advanced the art of resin binders that are suitable for forming prepreg mica tapes, the disclosed resins do not exhibit an optimized reactivity. Specifically, these resins exhibit some degree of reactivity at temperatures necessary to completely remove their volatile components. As a result, gelation tends to occur during the vacuum heat treatment cycle, preventing the elimination of voids and thereby degrading the effectiveness and life of the insulation. In the use of these resins, gelation is avoided only by carefully monitoring the hot vacuum cycle, thereby complicating processing. Furthermore, these resins have tended to react over extended periods at room temperature, such as periods in excess of one month, necessitating that they be refrigerated in order to promote their shelf life.
Finally, the resin compositions taught by U.S. Pat. Nos. 4,603,182 and 4,656,090 included styrene or vinyl toluene as diluents, which are reactive and volatile compounds, that can be removed during vacuum heat treatment, and thereby result in a variable product depending on how much was removed. Because these resin compositions have a tendency to gel during the hot vacuum cycle, such gelation hinders the ability to achieve adequate compaction in order to obtain a void-free insulation. Accordingly, the mechanical and electrical properties of a resulting cured insulation can be diminished.
In view of the above, it would be desirable if a resin binder were available that exhibited more optimal reactivity properties, specifically in terms of being: essentially unreactive at room temperature for storage stability; essentially unreactive at about 50.degree. C. to about 120.degree. C. in order to enable manufacture of the prepreg by hot melt soaking; essentially unreactive and having a low viscosity at a suitable vacuum heat treatment temperature so as to remove air, moisture and volatiles during processing of a conductor wrapped or taped with the sheet material; and highly reactive at practical curing temperatures. If such a resin binder were to exist, a substantial improvement could be achieved in the shelf life of mica tapes, the removal of volatile components during processing of the taped conductor, the avoidance of void formation during curing, and the effectiveness and life of the resulting insulation.